Hirise Mars mission - full well, gamma?

Discussion in 'Digital Photography' started by Ilya Zakharevich, Jul 10, 2007.

  1. I'm looking at what people write about the Mars caverns, and can't
    make head or tail of what they say... E.g.:

    What's amazing is that we cannot see any detail in the shadow

    is "stretched" to best see the darkest area, yet we still cannot see
    details except noise

    (see http://hirise.lpl.arizona.edu/PSP_003647_1745, and the "stretched
    version" in


    Well, I can easily see the details in the dark area (there are 3
    different levels of "gray" there), AND I do not see any noise to speak
    off. It is clear that a lot of shadow details is cut out by
    posterization (otherwise one would be able to see the noise).
    However, without knowing more about the image (what is the gamma,
    where was it converted to 8-bit - on-board vs on-the-ground, what is
    the full well of the sensor in the mode the shot was made, what kind
    of lookup tables they use) it is hard to make any deduction.

    What I found is that the read noise is about 80 electrons, the
    posterization is about 20 electrons/data-number, and that ADC has
    about 13bits usable. I can guess that the maximum full well is about
    128/8*26000 electrons. What about the rest?

    Is it correct that there is practically no illumination from the
    atmosphere diffuse lighting, and all the illumination of the bottom of
    the cavern comes from stars (so it is the "night" illumination)? Then
    what is the ratio of day/night illumination of the surface?

    Ilya Zakharevich, Jul 10, 2007
    1. Advertisements

  2. Ilya Zakharevich

    DeanB Guest

    Here's all I can get out of PSP32:


    No much in the way of detail, certainly nothing to be proud of or
    DeanB, Jul 10, 2007
    1. Advertisements

  3. [A complimentary Cc of this posting was sent to
    ??? Did you see that there is no noise, and there are 3 levels of gray?

    For me, this implies that there was a major posterization; AND this
    posterization could not remove all details. My impression is that by
    minor reprogramming, the shots can be repeated with a lot of details
    in the darks. Is it so? And if it is, where are the shots?

    Essentially, another of my questions is about calibration: one of
    these 3 levels is "background" (read: black); now these 2 levels of gray
    "above background", what is their actual brightness (in units of the
    surface illumination, and in units of night illumination).

    Ilya Zakharevich, Jul 10, 2007
  4. To answer your questions, you might try reading the team's research
    paper on the instrument, just published:


    Roger N. Clark (change username to rnclark), Jul 12, 2007
  5. [A complimentary Cc of this posting was sent to
    Roger N. Clark (change username to rnclark)
    It is not freely available. IAF_2004_Bergstrom_revH.pdf and 3287.pdf,
    which I read before posting, does not contain the necessary info.


    What else puzzles me, is the claim that "usually images of deep holes
    on Mars contain details due to star illumination". On Earth, sunlit
    is about 15ev, star illumination is about -6ev. On Mars, sunlit
    should be about 14ev; even assuming that star illumination is 1 step
    stronger (due to less atmospheric losses), this is 19 steps below the
    sunlit part. As claimed, the imager's ADC has about 13 usable bits;
    where the 6 other steps go?

    Ilya Zakharevich, Jul 12, 2007
  6. Gee, you can't afford $9?
    That is strange.
    If I get a chance, I'll ask Alfred, who I'll see in a couple of weeks
    at a meeting.

    Roger N. Clark (change username to rnclark), Jul 12, 2007
  7. [A complimentary Cc of this posting was sent to
    Roger N. Clark (change username to rnclark)
    Ilya Zakharevich, Jul 12, 2007
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.